Diglycidyl ethers of 2,2-bis(4-hydroxyphenyl)propane (abbreviated DPP or Bisphenol A or BPA) of a large variety in molecular weights are well-known commercial products. They are also known as epoxy resins, and find use on a large scale as main resinous components in curable or thermosetting compositions.
General reference to such epoxy resins is made in the books: "Handbook of Epoxy Resins" by Henry Lee and Kris Neville, dated 1967, and "Uses of Epoxy Resins" by W. G. Potter, dated 1975, as well as in a multitude of issued patents, such as, for example U.S. Pat. No. 2,633,458.
These epoxy resins are compounds having the general formula: ##STR1## wherein m is a number having an average value from 0 to 12. Compounds of the above formula wherein m is 0 to 1 are liquid or semiliquid at room temperature, compounds wherein m is at least 2 are solids at room temperature.
For some purposes, however, some properties of epoxy resins from DPP need improvement. For example, the viscosity of the liquid resins is rather high, usually 10 poise (25.degree. C.) or more. When a resin of lower viscosity is required, the DPP-based resin has to be diluted with a small amount of a reactive diluent, as, for example, a mono-epoxide or di-epoxide of low viscosity and other structure, usually a glycidyl ether of a monohydric alcohol or a glycol. These additives, however, will usually reduce the resistance of the cured products to heat, solvents, and/or chemicals.
Further, the glycidyl ethers of DPP have low compatibility for aliphatic hydrocarbons (which are attractive as cheap solvents), and have to be dissolved or diluted with the more expensive polar solvents, such as ketones, glycol ethers or glycol esters.
For some purposes, a better flexibility of cured products is required. This can be obtained by use of flexibilizers (non-reactive), flexibilizing epoxides, and/or flexibilizing curing agents. Apart from price considerations (the additives are usually more expensive) there is usually a reduction in other useful properties, such as resistance to heat, solvents, and/or chemicals.
Summarizing, there is still a need for epoxy resins of the bisphenol type, having good compatibility with aliphatic hydrocarbon solvents, and providing the usual curing agents cured products of improved flexibility, without undue reduction in adhesion, hardness, and resistance to heat and chemicals, whereas the liquid resins of that type should have a low viscosity.